There are many kinds of metal reducing processes for producing reduced iron or alloy steel. As a process with low cost and high productivity among the processes, an operation using a rotary hearth furnace (hereinafter, referred to as RHF) has been performed, and an outline of the process is described, for example, in Patent Document 1.
FIG. 1 shows a section of an RHF in a diameter direction. The RHF is a burning furnace (hereinafter, referred to as a rotary furnace) in which a refractory hearth 4 having a disk shape without a center on a wheel 3 rotates at a predetermined rate on a rail 5 describing a circle, under a ceiling 1 and a side wall 2 of a fixed refractory. The side wall 2 is provided with a plurality of burners 6, from which fuel and air are fed into control an atmosphere gas component and a temperature in the furnace. Generally, a diameter of the hearth of the rotary furnace is 10 to 50 meters, and a width thereof is 2 to 8 meters. A formed article as a raw material formed of powder including metal oxide and carbon is supplied onto the hearth 4, and is heated by radiant heat from upper gas in the furnace, thereby obtaining metal in the formed article by a reaction of the metal oxide and the carbon in the formed article.
FIG. 2 shows an example of the whole equipment of the RHF.
Metal oxide such as powdered ore and metal oxide dust is used as a raw material, and carbon is used as a reductant. Particle iron ore such as pellet feed as a metal oxide source, or a by-product material produced from an iron producing process such as converter dust, sintered dust, and blast gas dust is used in producing reduced iron. Coke, oil coke, coal, and the like are used as carbon that is a reductant. It is more preferable that a ratio of carbon powder that is not volatilized is more preferable, up to 1100° C. as a temperature at which a reduction reaction occurs. Such a carbon source is powder coke or anthracite.
In a ball mill 11 that is a mixing device shown in FIG. 2, powder including metal oxide and powder including carbon are mixed, and the mixture is formed into a granular form in a granulation device 12. The formed article is supplied onto the hearth 4 of the rotary furnace 13 to be uniformly laid. In the rotary furnace 13, the formed article is moved to each portion in the furnace by rotation of the hearth 4.
The formed article is heated at 1000° C. to 1500° C. by radiation of high temperature gas, and the metal oxide is reduced by the carbon in the formed article. Exhaust gas generated in the furnace is thermally collected in a boiler 15 and a heat exchanger 16 through an exhaust gas duct 14, dust is removed from the exhaust gas in a dust collector 17, and then the exhaust gas is discharged from a chimney 18 to the air. In the rotary furnace 13, the formed article is settled on the hearth 4, and thus there is an advantage that the formed article is hardly collapsed in the furnace. As a result, there is an advantage that there is no problem caused by attaching the raw material formed into powder on the refractory. In addition, there is an advantage that a powder raw material or a coal-based reductant with high productivity and low price can be used.
A metallization ratio of the reduced iron produced by such a method is generally 90% or less, and the maximum thereof is about 95%. The metallization ratio is relatively low as compared with directly reduced iron (hereinafter, referred to as DRI) produced by a gas reducing method such as a MIDREX method.
In the gas atmosphere in the RHF, carbon dioxide concentration is relatively high, and thus the furnace is not suitable for reduction substantially. However, since the iron oxide and the carbon are mixed in the formed article, an active reaction (FetO+C→tFe+CO) occurs in the formed article. Accordingly, there is reduction ability. As a result of the reaction, a ratio of carbon monoxide in the formed article and around the formed article gets higher, and a reduction property is atmospherically high around the formed article. Accordingly, the reduction of the iron oxide proceeds. However, when the ratio of metal iron in the formed article gets higher, a reduction reaction rate gets lower by the decrease of the ratio of the iron oxide. Accordingly, the ratio of the carbon monoxide in the formed article and around the formed article decreases. Therefore, when the metallization ratio is high, there is a problem that the reduction is delayed.
For example, as described in Patent Document 2, there is a method for producing the reduced iron with high strength, in which the high strength reduced iron is supplied to a blast furnace together with massive ore or sintered ore to produce pig iron. In this method, since preliminarily reduced iron oxide is finally reduced in a blast furnace, thermal load of the blast decreases. Accordingly, there are effects that coke consumption of the blast furnace decreases and production of pig iron increases.
A general method for operating a blast furnace using reduced iron has been performed from old times. For example, as described in Patent Document 3, a technique for using a large amount of reduced iron is disclosed. In case of using a large amount of reduced iron with high reduction ratio or scrap, there is described a technique of controlling a temperature in a furnace by controlling a blowing air temperature or an amount of blowing dust coal.
In a vertical furnace other than a blast such as a cupola, an operation for melting reduced iron together with scrap has been performed. For example, as described in Patent Document 4, massive coke and scrap are fed in a furnace, air or oxygen containing air heated from a lower part of the furnace is blown in, and the massive reduced iron (Hot Briquette Iron (HBI) or DRI) is molten together with the scrap in the production of melting the scrap, thereby producing pig iron.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2001-303115
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2004-218019
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2001-234213
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. H11-117010
[Non-Patent Document 1] “Dust Recycling Technology by the Rotary Hearth Furnace at Nippon Steel's Kimitsu Works”, Revue de Metall. Cahiers d'Inf. Tech. (2002) Vol. 99, (10), p. 809-818, T. Ibaraki and H. Oda